dropdown menu hidden by slideshow

Hi

I've set-up a site in D8, using D8W3CSS theme (thanks to fkelly ), for its dropdown menu that is ok for my needs. the drop down menu is in the "primary menu" region

Then I set-up a slideshow with Views_slideshow module, creating a view for a block that I put in the "highlighted" region, that is just below the "Primary menu"... this block is showing up only on the frontpage

the problem is that when I hover on the first level items of the dropdown menu, the children items are not shown because they are hidden by the slides of the slideshow block

the children items appear just for a fraction of a second during the transition between the slides.

obviously on the other pages, when the slideshow block is not shown, the dropdown menu is working as expected

I've googled a lot, trying to identify the z-index values of the element, to change them to a (very) higher value, setting the "overflow " to "visible ! important" .... no luck.. the submenu is always behind the slide

is there any other trick that I can use?

thanks!